The present invention relates to the field of wellbore drilling operations where telemetry is used. In particular, the invention relates to a method and system for wellbore drilling where cable based telemetry is used for communication between the surface and downhole devices.
Communication between downhole sensors and the surface has long been desirable. This communication is, for example, an integral part of methods known as Measurement-While-Drilling (MWD) and Logging-While-Drilling (LWD). Various methods that have been tried for this communication include electromagnetic radiation through the ground formation, pressure pulse propagation through the drilling mud, acoustic wave propagation through the metal drill string, and electrical transmission through an insulated conductor or cable arrangement. Each of these methods has disadvantages associated with signal attenuation, ambient noise, high temperatures and compatibility with standard drilling procedures.
The most commercially successful of these methods has been the transmission of information by pressure pulse in the drilling mud. However, attenuation mechanisms in the mud limit the effective transmission rate to less than 10 bits per second, even though higher rates have been achieved in laboratory tests.
Of the various alternatives to telemetry based on mud pulses, telemetry based on electrical transmission through an insulated conductor or cable arrangement has the considerable advantage of greatly increased data transmission rates.
There are a number of known methods that concern high-data-rate telemetry systems for MWD/LWD measurements using cables inside and/or outside the drillstring, however most suffer from severe limitations. Some techniques require a large number of connectors and are thus prone to reliability problems. Others require significant changes to operational procedures. The following examples of known methods will be briefly discussed.
U.S. Pat. No. 4,001,774 discloses a method wherein the communication link between a subsurface location in a well and the surface location is established and maintained through electromagnetic coupling between two insulated electric conductors locate insider the drillstring. The disadvantages of this type of arrangement include, due to hydraulic effects, risk of the loose end of the cable getting tangled. The method also relies on induction coupling which can be unreliable. Finally, the method requires that the cable be fished through each new pipe stand during drilling.
U.S. Pat. No. 4,098,342 discloses a technique that uses an insulated electric conductor inside the drillstring in a convoluted configuration to provide an excess length of conductor. The disadvantages of this type of arrangement include the problems associated with fishing the cable through each pipe stand, as well as the pulleys, weights and couplers exposed to the mud environment.
U.S. Pat. No. 4,143,721 discloses a technique of enabling an instrument and a connected flexible cable contained in a drillstring to be left inside the drillstring while a pipe section is being added to or removed from the drillstring. The disadvantages of this type of arrangement include relative vulnerability of the cable to damage, complexity in pipe handling, and the limitation of rotatability of the drillstring.
U.S. Pat. No. 4,153,120 discloses an arrangement using a cable inside a drillstring wherein the cable connection is temporarily broken when adding or removing pipe sections. A portion of the cable may be wound about an element within the drillstring, to be unwound therefrom each time the drill string is lengthened. The disadvantages of this type of arrangement include taking out the coil, unwinding and threading the cable which can be time consuming and costly.
U.S. Pat. No. 4,181,184 discloses that uses an all-internal kink resistant cable with a hanger fixed to the tool joints. The disadvantages of this type of arrangement include the need to fish the cable through each new stand of pipe; risk of tangling; and the increased pressure drop of the drilling mud due the decreased cross section available to carry mud.
U.S. Pat. No. 5,096,001 discloses technique wherein a sensor close to the drillbit in a deep small diameter section of the borehole is connected by cable within the drillstring to a mudpulse generator operating in an upper section of the borehole. The mudpulse generator transmits the signal to a receiver at the surface. The disadvantages of this type of arrangement include the limitation on data transmission rate due to the reliance on mud pulse telemetry. The system is also only capable of one-way transmission.
U.S. Pat. No. 4,057,781 discloses a method that combines the use of an inner cable and a drill pipe used as a conductor. The disadvantages of this type of arrangement include those associated with the use of an insulation coating on the casing.
U.S. Pat. No. 4,416,494 discloses a method using an internal cable that is stored within the drillstring as a coil in a flexible storage means. The disadvantages of this type of arrangement include the need to fish the cable through each stand of pipe.
Thus in light of the known techniques for telemetry, there is a need to an improved system and method.
According to the invention a system is provided for communication while drilling a borehole with a rotatable drillstring extending from a drilling rig at a top end to a drill bit attached to a bottom end, the communication system comprising a cable positioned along the outside of the rotatable drillstring for a portion of the length of the drill string, the cable being used for communication between the drilling rig and an assembly located in the borehole.
Also provided according to the invention is a method of communication while drilling a borehole between a drilling rig and an assembly in the borehole comprising the steps of:
drilling a borehole using a rotating drillstring extending from a drilling rig at a top end to a drill bit attached to a bottom end; and
positioning a cable along the outside of a portion of the rotating drillstring, the cable being used for communication between the drilling rig and the down-hole assembly.